c++ STL copy_backward question

int main () {
  vector<int> myvector;
  vector<int>::iterator it;

  // set some values:
  for (int i=1; i<=5; i++)
    myvector.push_back(i*10);          // myvector: 10 20 30 40 50

  myvector.resize(myvector.size()+3);  // allocate space for 3 more elements

  copy_backward ( myvector.begin(), myvector.begin()+5, myvector.end() );

 开发者_运维百科 cout << "myvector contains:";
  for (it=myvector.begin(); it!=myvector.end(); ++it)
    cout << " " << *it;

  cout << endl;

  return 0;
}

Why the output is "myvector contains: 10 20 30 10 20 30 40 50"

why not "30 40 50 10 20 30 40 50"

The implementation of copy_backward is here:

template<class BidirectionalIterator1, class BidirectionalIterator2>
  BidirectionalIterator2 copy_backward ( BidirectionalIterator1 first,
                                         BidirectionalIterator1 last,
                                         BidirectionalIterator2 result )
{
  while (last!=first) *(--result) = *(--last);
  return result;
}

So confused. thank you for all your help.

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That output looks right to me according to the way the code is written. You are copying from and into the same vector. You are copying from [begin, begin +5] (10 20 30 40 50) and you are copying to [end, end-5]. So 10 20 30 [10 20 30 40 50] is the right output for that code. The first 3 elements are untouched.

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If you want to copy something backwards, use reverse-iterators: rbegin() and rend(). Then just use the regular std::copy.